1. Field of the Invention
The present invention relates to a focus detection device that detects the focusing condition of a photographic lens, a photographic lens unit, an image-capturing apparatus and a camera system.
2. Description of Related Art
As disclosed in Japanese Laid Open Patent Publication No. 2007-121896, a focus detection device known in the related art, with the center of the entrance pupil set on the optical axis of a photographic lens, executes focus detection based upon a phase difference (image shift) manifested by a pair of pupil images on the image-capturing plane. In order to determine a defocus amount based upon the extent of image shift having been detected, the focus detection device calculates the distance between the gravitational centers of the pair of pupil images (pupil gravitational center distance).